This invention relates to an induction heating type of fixing device and an image forming apparatus equipped therewith such as copy machine, printer and facsimile equipment. More particularly, this invention relates to energization control of an induction heating type of fixing device.
The image forming apparatus is equipped with a fixing device that thermally fixes toner images on a recording material. An induction heating type of fixing device is equipped with an electroconductive heating roller including an induction coil as a heat source and a pressing roller that is pressed against the heating roller and forms a nip section.
The induction coil is as long as the maximum width of the recording material to heat the whole heating roller (maximum width of the recording material). This configuration has caused various problems such as uneven surface temperature of the heating roller along the roller axis and unwanted temperature rises at roller surfaces where a recording material of a smaller size (than the maximum width of the recording material) does not pass by. To solve such problems, some methods have been disclosed. One of such methods is dividing one induction coil into some parts and controlling the quantities of currents to respective induction coil parts according to the quantity of current to a preset induction coil part. Another method is controlling energization of respective induction coil parts independently.
For example, Patent Document 1 discloses a fixing device that controls currents to drive second and later excitation coils according to the current to drive the first excitation coil in order to keep the surface temperature of the fixing roller constant along its axis.
Patent Document 2 discloses an induction heating type of fixing device that is equipped with first and second temperature measuring means for detecting surface temperatures of a roller to be heated by first and second induction coils and controls driving currents for the induction coils independently to prevent excessive temperature rise at roller surfaces where a recording material of a smaller size does not pass by.
Patent Document 3 discloses an induction heating type of fixing device that synchronizes switching operations to turn on and off inputs to a plurality of induction coils, generates on-off signals by frequency modulation, and assigns a delay time for the turn-on signal of each induction coil.
Patent Documents 1, 2 and 3 mentioned above denote Japanese Non-examined Patent Publications 2000-206813, 2002-23557, and 2002-124369, respectively.
However, in the above prior art that energize a plurality of divided induction coils independently, it may sometimes happen that the induction coils may be energized simultaneously when operation modes or paper sizes are changed in the image forming apparatus.
When a high-frequency driving current flows through an induction coil that is made of a copper wire, the coil may generate a small vibration under the influence of a magnetic field. When the induction coils and the heating roller have different characteristics such as inductances and capacitances, the driving currents supplied from a power supply may have different frequencies. Therefore, if the adjoining induction coils are energized simultaneously, driving currents of different frequencies flow through the induction coils and cause the coils to generate small vibrations. These small vibrations turn into a big vibration by resonance. Further, this vibration resonates the heating roller and as the result, the induction coils generate noises such as resonant noises.
When the induction coils are not energized simultaneously, the temperature of the heating member may drop because of a loss time during which the heating member is not heated by the induction coils. This temperature drop causes a fixing trouble and a reduction in the heating rate of the heating member. Consequently, this prolongs the warm-up time (WUT: a time period required to heat up the heating member until it becomes ready to fix). Therefore, it is requested to control energization of induction coils without a loss time when heating the heating member.
Meanwhile, the image forming apparatus-has a maximum current rating (for ecxample, 15 A) and a maximum current available to the fixing device is limited by the operation mode (Image Formation mode, Warm-up mode, or Standby mode) of the image forming apparatus.
Judging from the above, we are requested to do the following when controlling energization of the fixing device: to avoid simultaneous energization of induction coils and to control energization more finely according to the operation mode of non-fixing devices in the image forming apparatus in consideration of the current supply balance of the induction coils.